Electronic Design

Composite Amp Has Low Noise, Drift

Typically, FET input operational amplifiers have desireable characteristics such as good frequency response, low input bias current, and low input-bias-current noise compared to bipolar input amplifiers.

On the other hand, bipolar operational amplifiers offer such noteworthy characteristics as low input offset voltage, low input-offset-voltage drift, and low voltage noise.

This composite amplifier circuit (see the figure) offers the best of both worlds. It achieves low current and voltage noise, combined with low input offset voltage and drift, without degrading the overall system's dynamic performance. Compared to a standalone FET input operational amplifier, the composite amplifier, circuit exhibits a 20-fold improvement in voltage offset and drift.

In this circuit arrangement, A11 is a high-speed FET input op amp with a closed-loop gain of 100 (the source impedance was arbitrarily chosen to be 100 kΩ). A2 is a Super β eta bipolar input op amp. It has good dc characteristics, biFET-level input bias current, and low noise. A2 monitors the voltage at the input of A1 and injects current to A1's null pins. This forces A1 to have the input properties of a bipolar amplifier while maintaining its bandwidth and low input-bias-current noise.

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